1. Field of the Invention
The present invention relates to a sealing film for a solar cell consisting chiefly of ethylene-vinyl acetate copolymer and a solar cell using the sealing film, particularly a sealing film and a solar cell having excellent productivity and a process for the preparation of the solar cell.
2. Description of the Related Art
A solar cell (solar cell module) has been paid attention as a device directly converting solar energy into electric energy from the viewpoints of effective use of natural resources and ecology, and various solar cells have been developed.
As shown in FIG. 1, a solar cell has a structure that silicon photovoltaic elements 4 (photovoltaic elements made of silicon etc.) are sealed by ethylene-vinyl acetate copolymer (EVA) sealing films 3A, 3B between a glass plate 1 as a transparent front side protection material and a backside protection material 2 (backside covering member). A sealing film arranged on light-receiving side of the photovoltaic elements is hereinafter referred to as a front side sealing film and a sealing film arranged on rear side of the photovoltaic elements is hereinafter referred to as a backside sealing film.
The solar cell is prepared by the following steps: a step superposing a glass plate 1, an EVA film (sealing film) 3A, silicon photovoltaic elements 4, an EVA film (sealing film) 3B and a backside covering member 2 in this order and a step curing or crosslinking them by heating to combine with each other.
In the solar cell, it is strongly demanded that light incident upon the solar cell is effectively taken into the photovoltaic elements of the solar cell as much as possible from the viewpoint of improvement of power generation efficiency. Hence, desired is the EVA film for sealing which has high transparency and therefore has properties transmitting almost incident solar light without absorbing and reflecting the solar light. Further when the solar cell is used for long time period, the EVA film is discolored due to influence of light and heat to show tendency of transmittance of sunlight to reduce. Therefore an EVA sealing film having excellent light resistance and heat resistance is demanded.
When a solar cell (module) is prepared, even the EVA film for sealing having excellent transparency, light resistance and heat resistance requires a long time period in a step for sealing the photovoltaic elements between the transparent substrates. The step is disadvantageous in view of productivity, and hence reduction of time period of the step is desired. In more detail, in a process for superposing a glass plate 1, a front side EVA film (sealing film) 3A, silicon photovoltaic elements 4, a back side EVA film (sealing film) 3B and a backside covering member 2 in this order and curing or crosslinking them by heating to combine with each other, the step for curing or crosslinking the EVA films requires tens minutes period, which is deemed to the cause requiring long time period for the preparation of the solar cell.
Generally, an EVA film can be rapidly cured or crosslinked by using an organic peroxide (crosslinker) having high reactivity due to reduced half-life temperature. However, the use of the organic peroxide (e.g., alkyl-peroxy ester, peroxy ketal) having reduced half-life temperature, the crosslinked EVA film is apt to generate blister with long-term use.
Patent Document 1 (JP 11-26791A) proposes that dialkyl peroxide and alkylperoxy ester or peroxy ketal are used in a ratio by weight of 10/90 to 90/10 as an organic peroxide (crosslinker). Further Patent Document 1 describes that the use of the organic peroxides brings about reduction of crosslinking time period, whereby a solar cell provided with a crosslinked film free from yellowing and blister can be obtained.
Patent Document 1: JP11-26791A